1. Field of the Invention
The present invention relates to a humidity measuring apparatus suitable for an image forming apparatus such as one of a copying apparatus and a printer using an electrophotographic system and, more particularly, to an error detection of such an apparatus.
2. Description of the Related Art
Generally, in an image forming apparatus of an electrophotographic system, a fluctuation occurs in concentration characteristics of a print image due to a using environment, a characteristics fluctuation of a developing device and a photosensitive drum depending on the number of print copies, a sensitivity variation of the photosensitive drum upon manufacturing, and a variation in frictional charging characteristics upon manufacturing of toner. Although an effort to stabilize those changes and fluctuating characteristics is being made everyday, it is insufficient yet. Particularly, in a color image forming apparatus, since a color image is reproduced by overlaying developers (toner) of four colors of yellow, magenta, cyan, and black, if concentrations of the developers of the four colors, that is, toner images are not accurately adjusted, a good color balance cannot be obtained. Therefore, in many color image forming apparatuses, the using environment is monitored and image forming conditions such as charging potential, exposure amount, and developing bias are changed to optimum values according to the using environment.
In the color image forming apparatus, generally, various humidity sensor elements are used to monitor the using environment. As a humidity sensor element, a humidity sensor element using such a phenomenon that an impedance changes due to an adsorption of water molecular is known. Such impedance change type humidity sensor elements include a ceramics humidity sensor element and a high molecular system sensor element.
The impedance change type humidity sensor element is further classified into an element using a resistance change (R change) and an element using an electrostatic capacitance change (C change).
An electrostatic capacitance of the electrostatic capacitance change type humidity sensor element (high molecular system sensor element) decreases at an almost same ratio according to an increase in humidity. The resistance change type humidity sensor element has a high resistance on the low humidity side and its resistance value exponentially decreases (RH-C characteristics become almost linear) according to the increase in humidity. The electrostatic capacitance change type humidity sensor element (high molecular system sensor element) has an excellent linearity and can measure the humidity from a relative humidity of 0% RH. However, the electrostatic capacitance at 0% RH is large to be hundreds of pF. A change width of the electrostatic capacitance at 0 to 100% RH is small to be tens of pF. Therefore, it is necessary to increase the small capacitance change and, at the same time, set off a large zero offset. There are, consequently, such problems that a circuit becomes very complicated, costs are high, and a periodic calibration is necessary.
According to the resistance change type humidity sensor element, since it is difficult to measure a low humidity area (5% RH or less) and a change width of the impedance shows exponential function characteristics of four to five digits, there is such a problem that it is difficult to assure a dynamic range of a circuit system. Further, there is such a problem that the characteristics fluctuation by a temperature is large. However, since such problems can be solved by devising the circuit system and additionally using a temperature detecting element, the resistance change type humidity sensor element is used in the invention. Circuit constructional diagrams for detecting the humidity have been proposed in Japanese Patent Application Laid-Open No. H02-298848 or Japanese Patent Application Laid-Open No. H07-311169.
According to Japanese Patent Application Laid-Open No. H02-298848 or Japanese Patent Application Laid-Open No. H07-311169, the circuit construction is devised so as to obtain good linearity characteristics in the whole humidity area in a range from the low humidity side to the high humidity side. However, since the circuit construction proposed in one of Japanese Patent Application Laid-Open No. 2-298848 and Japanese Patent Application Laid-Open No. H077-311169 is expensive, it is necessary to detect the humidity by a more reasonable circuit construction.
FIG. 1 (although it is a diagram of an embodiment, it is also cited in common to describe the related art) is a diagram illustrating a general circuit construction of a humidity detecting circuit for detecting the humidity by using an impedance change. In the diagram, a power source (for example, +3.3V) is supplied to a microprocessor mounted on a board. A signal of a predetermined frequency (for example, 1 kHz), an amplitude (for example, +3.3V), and a duty ratio (for example, 50%) is output from the microprocessor. An output signal CLK and an output signal /CLK whose polarity is opposite to the polarity of the signal CLK are supplied to a humidity sensor element 101 through a voltage dividing resistor 102. A value of the voltage division with the resistor 102 is input to the microprocessor.
FIG. 15 is a diagram illustrating a signal waveform of each section in FIG. 1. The CLK signal and the /CLK signal are output from the microprocessor. A sensor signal SNS is A/D-input. Timing for the A/D input is detected after the elapse of a specified time while setting a leading edge of the CLK signal to a reference. The humidity is calculated from a detection voltage of the sensor signal SNS by using a detection voltage−humidity conversion table.
However, the above related arts have the following problems.
FIG. 16 illustrates signal waveforms at the time of the high humidity and the low humidity when using the circuit illustrated in FIG. 1. In the related arts, since the impedance change width of the humidity sensor element 101 in the range from the low humidity side to the high humidity side shows the exponential function characteristics, a dynamic range of a detection voltage Vi (refer to FIG. 16) is wide. That is, in the signal waveforms at the time of the high humidity and the low humidity, a fluctuation in the voltage which is detected depending on the humidity change is small. For example, when the humidity sensor element fails due to a disconnection or the like, a detection voltage value is the same as the value at the time of one of the high humidity and the low humidity and does not fluctuate. Therefore, at the time of one of the high humidity and the low humidity, it is difficult to detect from the A/D input value of the sensor signal SNS whether a cause of the failure is based on an abnormality of the humidity sensor element 101 or based on one of the high humidity and the low humidity. That is, the abnormality of the humidity sensor element 101 cannot be accurately detected. Thus, even when the humidity sensor element 101 is abnormal, the cause is determined to be based on one of the high humidity and the low humidity, the using humidity is erroneously detected, and abnormal values are set into the image forming conditions such as charging potential, and exposure amount, developing bias.
The invention is made under such circumstances and it is an object of the invention to provide an image forming apparatus which detects an error of a humidity measuring apparatus by a reasonable construction and can continuously provide images of high quality.